The present invention relates to a radical cell device used for enhancing the reactivity of substances by making a gaseous substance plasmatic in a MBE (Molecular Beam Epitaxy) apparatus and a sputtering apparatus, and a method of manufacturing a Groups II-VI compound semiconductor device. More particularly, the present invention relates to such a radical cell device that an impurity is not mixed up into the semiconductor layer from the radical cell device, for example, when the semiconductor layer such as ZnO based oxide compound or ZnSe which is to be a p-type semiconductor by utilizing nitrogen as a dopant is grown and a method of manufacturing a Groups II-VI compound semiconductor device using the radical cell device.
A blue color based (which means the wavelength region from ultraviolet to yellow, hereinafter, indicating the same meaning) light emitting diode (hereinafter, referred to as LED) used for a light source of a full color display, a signal light and the like and a blue laser (hereinafter, referred to as LD) used for high precision DVD light source of the next generation which successively oscillating at the room temperature have been recently obtained by depositing GaN based compound semiconductor layers on a sapphire substrate and have got into the limelight.
On the other hand, recently, studies on a blue color based LED and LD using ZnO based oxide semiconductor have been also proceeded. In this case, as a growth method of ZnO based oxide semiconductor layer, MOCVD (Metal Organic Chemical Vapor Deposition) method, MBE (Molecular Beam Epitaxy) method, LA (Laser Ablation) method and the like are considered, however, to date, the MBE method is the most suitable method to laminate layers having different components, each of which is an uniformed component for a light emitting device.
In a MBE method, as a schematic diagram of a MBE device is shown in FIG. 6(a), ZnO is epitaxially grown by irradiating, for example, raw materials from a material source 21 of Zn and a radical source 23 of oxygen toward a substrate 8 consisted of sapphire and the like. At this time, in the case where it is made of an n-type semiconductor layer, while irradiating Al from a material source 22 of Al, and in the case where a p-type semiconductor layer is made, ZnO is epitaxially grown by irradiating nitrogen from a radical source 24 of nitrogen.
In a device shown in FIG. 6(a), a main chamber 1 is a conventional chamber of a MBE apparatus and it is a container capable of maintaining ultra-high vacuum in a cylindrical shape and connected to an air escape (not shown). Then, within it, a substrate holder 4 for holding the substrate 8, on which semiconductor layers are grown, is provided so that the substrate 8 can be heated by a heater 5. And, material sources (cell groups) 21 through 24 for the materials constituting compound semiconductor layers which grows on the substrate 8 are provided as opposing to the substrate 8 held by the substrate holder 4. The material sources include radical cells which supply the material of gases such as oxygen, nitrogen and the like.
Each of the material sources 21 and 22 consists of, for example, a crucible provided with a shutter (not shown) on the front face as well as a heater (not shown) for being capable of evaporating of the raw material on the periphery of the crucible ,and the desired material is supplied to the side of the substrate 8. Moreover, in radical cells 23 and 24, an ECR (Electron Cyclotron Resonance) generating plasma, for example, using a microwave is configured so that oxygen and nitrogen excited by plasma are irradiated.
As a schematic diagram nearby a plasma radiation outlet 23a is shown in FIG. 6(b), in this radical cell 23, electrodes 23c and 23d for supplying a high electric field for ion trapper are provided nearby the irradiation outlet 23a of the plasma 23b so that a semiconductor layer grown is not influenced by a charged particle in plasma directly colliding the substrate. Specifically, for example, when oxygen is dissolved into a form of oxygen atoms by plasma, since the energy of a plasma excitation is high, charged particles such as ions such as O2 ion, O ion, a large capacity of electron ray and the like occur. If these charged particles are irradiated on the substrate, since such harmful influences that the surface of the substrate is charged, the growth of crystal is hindered and ZnO deposited is etched and the like are exerted and causes the occurrence of defective crystal, the semiconductor layer having a good crystallinity cannot be obtained.
In most cases, a conventional radical cell device may be provided with electrodes for applying a high electric field nearby the plasma radiation outlet in order to remove the charged particles from the plasma generated. Since this high electric field is on about 100 to 600 V, the electrode of the grounding side is provided so as to directly electrically contact with the metal plate to which the cell is grounded, and the electrode of the high voltage side is provided via insulation porcelain consisted of alumina (not shown).
In a MBE device, for example, when ZnO oxide semiconductor layer is grown, even if it is grown in an undoped manner, there is a problem that it easily becomes an n-type layer and a layer whose n-type carrier concentration is small cannot be obtained, and if it is intended to make a p-type layer, there is a problem that a sufficient high doped p-type layer cannot be formed and the carrier concentration cannot be precisely controlled.
The present inventors have made every efforts to sufficiently control the carrier concentration of an undoped ZnO layer, as a result, have found that Al is mixed up into the ZnO layer, although ZnO layer is made grown in an undoped manner, and which is one of the factors for making it n-type. Specifically, as shown in FIG. 5, as a result of the depth profile of Al in this ZnO layer having being examined by a SIMS analysis method, the existence of Al spanning over the entire growth thickness region of ZnO has been recognized. Then, the present inventors have further every efforts to find the cause of it, as a result, the present inventors have found it is caused by the fact that Al contained within ZnO layer is doped after Al is made free from the insulation porcelain constituting an ion trapper of a radical cell.
The present invention has been made in consideration of these circumstances, and an object of the present invention is to provide a radical cell device not so as to blow-off and mix up Al into the film even in the case where a thin film is made grown.
Another object of the present invention is to provide a method of manufacturing a Groups II-VI compound semiconductor device such as ZnO based compound semiconductor light emitting device whose emitting light characteristic is excellent, in which, ZnO based compound layer has the high purity without containing undoped Al.
As described above, the present inventors have found that when ZnO based compound semiconductor (which means an oxide containing Zn, and as a concrete example, besides ZnO, meaning ones including the respective oxide of Group IIA and Zn, Group IIB and Zn, or Group IIA and Group IIB and Zn, hereinafter, indicating the same meaning) is grown, even in the case where Al as a dopant is not doped, Al is doped in a comparatively high concentration in the ZnO based compound semiconductor layer, and the insulation porcelain for holding an electrode of ion trapper (charged particle removing means, hereinafter, indicating the same meaning) is the causative factor.
Specifically, it was found that the reason was followings. An electrode of ion trapper is mounted on the metal wall of the plasma chamber as described above via an insulation porcelain, charged particles are curved towards the electrode side by pulling by the electrode of the high electric field, the curved charged particles flow into the electrode and one portion of the curved charged particles also collides the insulation porcelain on which the electrode is mounted as well, and Al of the insulation porcelain is pulled out by its collision, reaches on the substrate surface along with plasma and is made doped within the thin film.
A radical cell device according to the present invention comprising, a plasma chamber in which closing plates are provided on both end portions of a cylindrical body and a gaseous substance is made plasmatic within the plasma chamber, a gas introduction tube which is inserted into one plate of the closing plates for introducing into the plasma chamber a gaseous substance, a plasma radiation outlet provided on the other plate of closing plates, and an electrode of an ion trapper for applying a high electric field, the electrode being provided nearby the passage of plasma outgoing from the plasma radiation outlet, wherein the electrode for applying a high electric field is fixed on a grounded metal plate via an insulation porcelain made of magnesium oxide or quartz.
By thus configuring the radical cell device, the high electric field is applied to the passage of plasma (plasma jet) outgoing from the plasma radiation outlet. And even the charged particles are pulled towards to the electrode by which a high electric field is applied and collide the electrode and the insulation porcelain nearby the electrode, since neither of the electrode and the insulation porcelain contains Al, Al being made free and Al being doped are largely reduced. It should be noted that even if Mg of MgO is made free and mixed up into, since Mg is an element of Group II, and Mg has no harmful influence on the Groups II-IV compound. Moreover, Si of quartz is an element of Group IV, however, even if a radical cell is configured by quartz as an insulation porcelain and an ion trapper is made acted, it has been recognized that little Si is doped and has no influence on the carrier concentration and the like.
Concretely, this is a structure in which the grounded metal plate is mounted on the other plate on which the plasma radiation outlet is provided, the electrode for applying a high electric field is fixed on the metal plate via the insulation porcelain so as to be opposed to a grounded electrode provided and electrically connected to the grounded metal plate, in which the passage of plasma is interposed between them.
Another concrete structure is one in which a metallic protective cover is provided around the plasma chamber, a shutter for opening and closing is provided on a portion corresponding to the passage of plasma of the metallic protective cover, the shutter is shared with the electrode for applying a high electric field, and the shutter is fixed on the protective cover via the insulation porcelain, and in the structure in which the insulation porcelain is formed by quartz or magnesium oxide (MgO).
A method of manufacturing a Groups II-VI compound semiconductor device according to the present invention is characterized in that in the case where a Groups II-VI compound semiconductor layers capable of containing a dopant for light emitting layer forming portion are made grown on the substrate by a MBE method, wherein when the constitutive element or dopant element of the compound semiconductor layers is introduced in a plasmatic condition, the plasmatic condition is generated by using a radical cell device which has an insulation porcelain made of magnesium oxide or quartz for holding an electrode for applying a high electric field of an ion trapper.